1. Field of the Invention
The present invention relates to a light source unit including a light source and a photodetector, and to a thermally-assisted magnetic recording head provided with the light source and the photodetector.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in recording density, and thin-film magnetic heads and magnetic recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head including a magnetoresistive element (hereinafter, also referred to as MR element) intended for reading and a write head including an induction-type electromagnetic transducer intended for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
Magnetic recording media are discrete media each made of an aggregate of magnetic fine particles, each magnetic fine particle forming a single-domain structure. A single recording bit of a magnetic recording medium is composed of a plurality of magnetic fine particles. For improved recording density, it is necessary to reduce asperities at the borders between adjoining recording bits. To achieve this, the magnetic fine particles must be made smaller. However, making the magnetic fine particles smaller causes the problem that the thermal stability of magnetization of the magnetic fine particles decreases with decreasing volume of the magnetic fine particles. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the magnetic recording medium, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the aforementioned problems, there has been proposed a technique so-called thermally-assisted magnetic recording. This technique uses a magnetic recording medium having high coercivity. When writing data, a magnetic field and heat are simultaneously applied to the area of the magnetic recording medium where to write data, so that the area rises in temperature and drops in coercivity for data writing. Hereinafter, a magnetic head for use in thermally-assisted magnetic recording will be referred to as a thermally-assisted magnetic recording head.
In thermally-assisted magnetic recording, near-field light is typically used as a means for applying heat to the magnetic recording medium. A known method for generating near-field light is to use a plasmon generator, which is a piece of metal that generates near-field light from plasmons excited by irradiation with laser light.
The laser light to be used for generating the near-field light is typically guided through a waveguide provided in the slider to the plasmon generator disposed near the medium facing surface of the slider. Possible techniques of placement of a light source for emitting the laser light are broadly classified into the following two. A first technique is to place the light source away from the slider. A second technique is to secure the light source to the slider.
The first technique requires an optical path of extended length including such optical elements as a mirror, lens, and optical fiber in order to guide the light from the light source to the waveguide. This causes the problem of increasing energy loss of the light in the path. The second technique is free from the aforementioned problem since the optical path for guiding the light from the light source to the waveguide is short.
U.S. Patent Application Publication No. 2008/0043360 A1 discloses a thermally-assisted magnetic recording head that uses the aforementioned second technique. The thermally-assisted magnetic recording head includes a slider and a light source unit. The slider has a waveguide with its incidence end face located in a surface of the slider opposite to the medium facing surface. The light source unit includes a light source support substrate, and a laser diode as a light source disposed on the light source support substrate. The light source unit is secured to the surface of the slider opposite to the medium facing surface so that laser light emitted from the laser diode is incident upon the incidence end face of the waveguide.
The laser diode, which is a semiconductor element, has such a property that even at the same power applied thereto, the intensity of emitted light significantly varies with varying ambient temperatures or variations in the temperature of the laser diode caused by its own heat generation. The variation in the intensity of emitted light from the laser diode leads to unfavorable variations in the intensity of the near-field light for the thermally-assisted magnetic recording head. For this reason, the thermally-assisted magnetic recording head needs to be designed to detect in real time the intensity of emitted light from the laser diode, and suppress the variation in the intensity of the emitted light on the basis of the detection results.
A stripe laser diode having a stripe-shaped light propagation path includes a first emission part for emitting forward light and a second emission part for emitting backward light. In the thermally-assisted magnetic recording head, the laser diode having the aforementioned first and second emission parts can be employed as a light source, with the forward light employed as the laser light for generating the near-field light. Then, the intensity of the backward light can be detected to exercise feedback control on the intensity of the emitted light (the forward light and the backward light) from the laser diode on the basis of the detection results.
In order to realize the aforementioned feedback control in thermally-assisted magnetic recording heads, the present inventors have proposed a light source unit that includes a unit substrate having a light source mount surface and incorporating an embedded photodiode, and a laser diode mounted on the light source mount surface of the unit substrate. In the light source unit, the photodiode has a light receiving surface that is located in the same plane as the light source mount surface at such a position as to be able to receive the backward light from the laser diode.
In the thermally-assisted magnetic recording head with the light source unit including the aforementioned photodiode, the reaction speed of the photodiode and the response speed of the feedback control dependent thereon increase with increasing amount of light received by the photodiode. Therefore, in order to increase the response speed of the feedback control, it is necessary to allow the photodiode to receive an increased amount of light. For the aforementioned light source unit, however, the center of the backward light, which is divergent light, is parallel to the light receiving surface of the photodiode. This allows only part of the backward light to be incident upon the light receiving surface. To be worse, this configuration causes the backward light to be incident upon the light receiving surface at a large incident angle, thereby causing the light receiving surface to exhibit a high reflectivity for the backward light. For these reasons, the amount of light to be received by the photodiode is decreased. In order to enhance the response speed of the feedback control under this situation, it is desirable either to increase the amount of light to be received by the photodiode or to somehow improve the efficiency of photoelectric conversions by the photodiode.
To increase the amount of light to be received by the photodiode, a possible approach is to increase the ratio of the intensity of the backward light to the intensity of the forward light. However, this approach disadvantageously causes a reduction in the intensity of the forward light that is used for generating the near-field light.
Another possible approach to increasing the amount of light to be received by the photodiode is to increase the size of the photodiode. In thermally-assisted magnetic recording heads, however, the photodiode cannot be greatly increased in size because the light source unit to be secured on the slider should not be large. It is thus difficult to increase the amount of light to be received by the photodiode by increasing the size of the photodiode.
JP-A-2010-225798 discloses a technique where a surface plasmon resonance (SPR) inducing area made of, for example, a matrix-shaped metal film, is provided on the surface or inside of a photoelectric conversion semiconductor device (solar battery). The SPR inducing area induces surface plasmon resonance to generate photoelectric field enhancement effects. This technique aims to improve the efficiency of photoelectric conversions in the near-infrared region of the solar battery.
However, according to the technique disclosed in JP-A-2010-225798, the SPR inducing area is not intended to receive light incident thereon in a particular direction, such as the backward light which impinges on the light receiving surface of the photodiode in the light source unit having the aforementioned photodiode.